The invention relates to a method for the optical study of structured surfaces of objects, especially of wafers and/or masks, with an observation beam path whose central axis is directed perpendicularly against the surface of the object, with an illuminating beam whose central ray falls perpendicularly on the surface of the object, and with an illuminating beam whose central ray falls obliquely on the surface of the object, while in the observation light path the image of the surface of the object is observed and/or detected, and to an apparatus for the practice of this method.
In optical inspection technology, complex structures on flat substrates are inspected in an image field. This is the case especially in the semiconductor industry for the optical inspection of the structured surfaces of wafers and masks. Any defects present, for example, are to be detected and classified and/or extremely small textural widths (xe2x80x9ccritical dimensionsxe2x80x9d) are to be examined and measured. Defects on the structured surfaces may be, for example, grains of dust, small air inclusions in the resist, resist residues on the wafers, broken edges, etc.
The study is performed, for example, with an optical apparatus and with illumination in which the central ray of the illuminating beam falls perpendicularly onto the surface of the object. Such an inspection device is, for example, embodied in a microscope with a Kohler bright field illuminating system.
It has been found, however, that the detection of defective edges, point defects, defects at corners and boundaries of raised and recessed structures with a bright-field illumination often is insufficient. For this reason recourse has been taken to an additional inspection with an illuminating beam whose central ray strikes obliquely onto the surface of the object. Such illumination is achieved, for example, in a microscope with a dark field illuminating system. This illumination is especially suited for the detection of the defective structures described. In the case of this dark field illumination, however, no surfaces are visible. The edges and structures appear in rich contrast as bright lines on a dark background. Irregularities in these lines indicate a defective edge or structure.
Illumination apparatus for a microscope are disclosed in DE-OS 20 21 784 and DE 23 31 750 C3, in which it is possible to shift as desired from bright field to dark field illumination. In both illumination apparatus, in the illuminating beam there are provided, among other things, a light source, adjustable diaphragms, an annular diaphragm with a center stop that can be turned in and out, and an objective with an annular mirror around the objective. The annular diaphragm is transparent and the center stop is opaque. By turning in the center stop the change is made from a bright field to a dark field illumination. The light is no longer thrown on the object through the objective, but only through the annular mirror. The central rays of the illuminating beam no longer strike perpendicularly onto the surface of the object but strike it obliquely. Both microscopes are equipped each with a common observation light path for the bright field and the dark field illumination.
In neither of the disclosures is any simultaneous bright field and dark field illumination provided.
EP 0 183 946 B1 discloses a combined bright field/dark field illumination device with two light sources, in which the change from bright field to dark field illumination is performed through mechanical locks. One lock is associated with each light source. In this device it is also provided such that both kinds of illumination are applied simultaneously. For this purpose both locks are opened. But it not stated in this disclosure what special advantages result from this mixed lighting. No coding and later decoding of the bright field and/or the dark field illumination beam takes place.
DE 37 14 830 A1 discloses a combined bright field/dark field reflected light illumination for a microscope in which the change from bright field to dark field illumination is performed by an insertable center stop. Furthermore, in the dark field beam path an optical element of annular construction is provided with individual lens crown surfaces arranged side by side. These lens crown surfaces can have different spectral transmittance. In this illuminating device, however, no simultaneous bright field and dark field illumination is provided. Here, again, it is not stated what special advantages result from such colored illumination.
An optimized inspection method for the optical testing of structured surfaces of objects with a microscope having a combination of bright field and dark field illumination uses the two illumination methods one after the other. This resulting manual changing of the lighting conditions not only causes a doubling of the time is required for image capture and inspection, but also an additional manual manipulation of the center stop on the microscope.
It is therefore the object of the present invention to provide a method and an apparatus for the optical inspection of structured surfaces of objects, in which the measuring and inspection time with a bright field illumination beam and a dark field illumination beam is minimized.
This object is achieved in the method of the invention in that at least one of the two illuminating beams is provided with coding and the surface of the object is lighted simultaneously by both illumination beams, and in the observation light path the images produced by the different illumination beams are separated from one another and presented for observation and/or evaluation.
This object is achieved in the apparatus of the invention in that an optical system with a filter device and/or detector device and an illumination device arranged in the observation light path is provided for the simultaneous generation of a bright field and a dark field illumination, a system being associated with the bright field and/or the dark field illumination beam for the purpose of coding to distinguish the illumination beams.
Additional advantageous embodiments of the invention are described hereinafter.
With the invention it is accomplished that the object is illuminated simultaneously with a bright field beam and a dark field light beam. At least one of the two beams is coded by color, polarization or modulation. The separation of the illumination beams reflected from the object takes place in the common observation beam path through a corresponding filter device and/or a corresponding detector device.
For example, the prepared object can be illuminated in the dark field at a low angle of incidence, with red light for example, and simultaneously with green/blue within the aperture cone of the objective, with the remaining visible color spectrum. Since the light in the common observation beam path is again mixed in the colors, a separation by color takes place in the observation light path, e.g., by a dichroic splitter followed by imaging to separate black-and-white CCD cameras.
An RGB-CCD camera can also be provided directly in the observation light path. The two light paths are then separated by a computer-based processing of the RGB channels.
In another embodiment of the invention, the illuminating apparatus has a common light source or at least two separate light sources for the simultaneous production of the bright field and dark field illumination.
In a simple embodiment of the invention, the surface of the object is illuminated in the bright field with normal white light and the oblique illumination beams of the dark field are created by a single light source which emits red light and is positioned laterally on the object. The separation of the two images that follows is performed in the observation beam path with a filtering and/or detecting apparatus.
The apparatus for coding the illumination beam can have, for example, a color filter, a polarizing filter, a modulation filter or a dichroic splitter. However, provision is also made for the use of colored light sources or light sources with a one-color or monochromic emission characteristic. The associated filter set-up can be equipped, for example, with a color filter, a polarizing filter, a demodulation filter or a dichroic splitter.
The detector device can also be configured such that at least one CCD element is present, which is configured as a black-and-white or color camera. In the case of an RBG camera and a color identification of the illuminating beams there is no necessity of an additional filter device for the separation into a bright-field or dark-field image, since the RGB channels can be read separately.
The detector device in an additional embodiment of the invention is connected electrically or electronically to a computer system. The computer system then has a plurality of computers operating in parallel for the simultaneous detection and/or evaluation of the images. With the computers working in parallel, different processing steps of image detection, image analysis or inspection, error analysis, error classification and metrology (measurement of structure widths) can be performed simultaneously. Parallel processing accordingly reduces the inspection time for a single object.
In another embodiment of the invention the optical system can be in the form of a microscope. A Kohler illumination system is advantageously provided in the microscope. This illumination system can be configured as a top lighting or substage lighting system.
In another embodiment of the invention, several differently coded dark field beam paths can be provided. The differently coded dark field beam paths can also illuminate the object from different angles.